A wideband mobile communication system using complex modulation techniques, such as wideband code division multiple access (WCDMA) and orthogonal frequency division multiplexing (OFDM), has large peak-to-average power ratio (PAPR) specifications and hence requires highly linear power amplifiers for its RF transmissions. The conventional feedforward linear power amplifier (FFLPA) has been widely utilized due to its excellent linearity performance in spite of poor power efficiency.
Conventional FFLPAs are mainly based on the principle of error subtraction and power-matching with dedicated hardware circuitries to realize nonlinear corrections to PA. These approaches must use an auxiliary PA and complicated hardware circuitries to match exactly the transmitted power-balance, time-delay and errors generated by the main PA. After a perfect matching is obtained, the non-linear distortion errors from the main PA can then be canceled by those distortion errors from the auxiliary PA. Due to the complexities of the nonlinear predistortion circuits, which among other things involve many variables and parameters, FFLPAs require significant fine tuning and other calibration efforts. In addition, such traditional FFLPA schemes are also vulnerable to fluctuating environmental conditions, such as temperature and humidity changes, since perfect alignment of the main PA's signal and that of the auxiliary PA are vital. As a result, traditional predistortion schemes are costly to implement and are limited in their predistortion accuracy and stability in a commercial wireless system environment.
In order to overcome the FFLPA's poor efficiency, digital baseband predistortion (PD) has been demonstrated due to the recent advances in digital signal processing (DSP) technology. In addition, Doherty power amplifiers (DPA) have also been applied to these linearization systems to improve power efficiency. However, there is still a demand for higher performance of the power amplifier such as more linearity and better efficiency with less expensive architecture.
Conventional DSP-based PD schemes utilize digital microprocessors to compute, calculate and correct the PA's nonlinearities, typically by performing fast tracking and adjustments of signals in the PA system. However, conventional DSP-based PD schemes are challenged by variations of the linearity performance of the amplifier due to changes in the environment such as temperature and the asymmetric distortions of the output signal of the PA resulting from memory effects. All of these variations and distortions have to be compensated for. Conventional PD algorithms are based on a wideband feedback signal, and require a high speed analog-to-digital converter (ADC) in order to capture the necessary information. In addition, time-synchronizations are typically required to capture an error signal between a reference signal and a distorted signal. This time-matching process may result in small synchronization errors which can further affect conventional PD schemes' linearization performance. Amplitude and phase synchronization is also required in order to align the reference signal and the distorted signal.
Moreover, conventional PD schemes necessitate coded in-phase (I) and quadrature (Q) channel signals in the baseband as the required ideal or reference signals. As a result, conventional PD schemes are often standard or modulation specific and must be closely tailored to each baseband system. Therefore, in order to deploy conventional PD schemes into base-stations, the PD engines must be embedded into the baseband architecture of base-stations. This embodiment is a practical implementation challenge since it is frequently inconvenient or impossible to modify the baseband architectures of existing base-stations or base-station designs. Once the PD scheme is set up for a specific base-station design, it is often not reconfigurable and hence not upgradeable to future changes in standards or modulations. Furthermore, since traditional PD approaches require baseband I-Q signal sources to operate, they are inapplicable to certain RF systems that do not possess any baseband I-Q signal sources, such as repeater and indoor signal coverage sub-systems.